Referring to FIG. 1, a purpose of RADAR is to accomplish detection, localization, and potentially classification of objects remotely. In active RADAR, a wave of electromagnetic energy 2 is launched from a transmission source 1a which derives the signals to be carried by the wave 1b from a RADAR Signal Processor 7. The transmitted wave propagates to the object (the “target”) 3 and is reflected or scattered by the object, returning an echo wave 4 to the RADAR Receiver 5. The receiver may, but need not, be co-located with the point of transmission.
Assume an initial wavelength of the transmitted wave to be λ0. Due to the well-known Doppler Effect, the wavelength is altered due to relative motion along a line between source and target, and by relative motion along a line between the target and the receiver 5 so that it will be received with a potentially different wavelength (λ). Due to the invariance of the speed of electromagnetic propagation (discounting for the moment the slight effect of index of refraction in a medium, such as air, which we assume to be very close to unity), the frequency of the wave is therefore altered. This alteration is referred to as the “Doppler Frequency Shift”. By determining the Doppler Frequency Shift present in received signal 6, the relative motion of the target may be inferred. In addition, the finite speed of light c, causes an observable time delay to be present in receiving the echo from target, which allows distance to the target to also be inferred. The processing needed to accomplish the measurements of time delay and frequency shift (and therefore range and range rate to the target) are accomplished in a RADAR Signal Processor 7.
A target may include multiple scatterers (e.g., a flock of migrating birds, weather features, etc.) creating multiple range and Doppler returns, which can be interpreted to allow additional classification of the target. Results of these measurements are supplied to an information processor and display subsystem 8 which assimilates the signal processor measurements, and interprets and displays the results in a useful form.
Localization of the target, even to the point where target imaging is possible, is important to RADAR. In current RADAR systems, it is often the case that the transmission and reception antennas use multiple elements that can be processed with phased-array (beam-forming) techniques to gain resolution in angle so that this can be accomplished. Synthetic Aperture RADARs and Inverse Synthetic Aperture RADARs makes use of relative motion to create longer apertures “synthetically” through processing of the coherent signal returns. The discussion above can be generalized for the case where the electromagnetic wave is in the infra-red or optical band of the EM spectrum (or even higher in frequency). For example, the optical case is termed LIDAR (Light (Wave) Detection and Ranging vs. Radio (Wave) Detection and Ranging). For the case where the wave utilized is non-electromagnetic, e.g. acoustic, the systems are referred to as SONAR (Sound Navigation and Ranging), SODAR (Sound Detection and Ranging), or Medical Ultrasonic Echo-Ranging/Ultrasonography. The disclosure here applies to all such generalized cases of the particular RADAR examples presented here.
The present disclosure applies as well to what is termed “passive” radar, where the object (target) carries a RADAR signal source and therefore only RADAR receivers are required (all transmitters are external to the device). FIG. 2 shows the passive radar concept.